In view of the concern that energy problems and environmental problems will become serious in the future, development of alternative energies that replace fossil fuels has been eagerly carried out. Among the candidates for alternative energy sources, a photoelectric conversion device that converts light into electricity by using a photoelectric effect inside of a semiconductor is attracting attention, and a thin-film photoelectric conversion device using a silicon-based thin-film as a photoelectric conversion layer has been widely researched and developed.
The photoelectric conversion layer is a layer that absorbs light to generate an electron and positive hole pair, and thus its absorption property correlates well with an electrical generation property of the thin-film photoelectric conversion device. For example, when a silicon-based thin-film is used as the photoelectric conversion layer, the absorption of light in the photoelectric conversion layer is not sufficient for a wavelength longer than 1000 nm, so that the power generation efficiency of the thin-film photoelectric conversion device decreases considerably. On the other hand, solar light that lands on the Earth's surface contains light having a wavelength longer than 1000 nm, so that there has been demand for development of a thin-film photoelectric conversion device capable of efficiently performing photoelectric conversion for light having a wavelength longer than 1000 nm and for making a more efficient thin-film photoelectric conversion device.
With respect to the thin-film photoelectric conversion device, Non-Patent Document 1 discloses a single-junction photoelectric conversion device using a weak n-type microcrystalline germanium as a photoelectric conversion layer in an attempt to improve a photoelectric conversion efficiency for light having longer wavelength. The thin-film photoelectric conversion device has a structure in which a stainless substrate/n-type amorphous silicon/i-type amorphous silicon/a gradient composition layer of microcrystalline silicon germanium/a weak n-type microcrystalline germanium photoelectric conversion layer/a gradient composition layer of microcrystalline silicon germanium/a p-type microcrystalline silicon layer/Indium Tin Oxide (ITO) are successively stacked. The characteristics of the thin-film photoelectric conversion device are: open circuit voltage Voc=0.22 V, short circuit current density Jsc=25 mA/cm2, fill factor FF=0.36, and conversion efficiency Eff=2.0%, and the wavelength that exerts a quantum efficiency of 10% on a longer wavelength side is about 1080 nm, with the wavelength that exerts a quantum efficiency of 5% being set to 1130 nm. The microcrystalline germanium photoelectric conversion layer is formed by using an ECR remote plasma-enhanced CVD method that uses a microwave discharge.